Electrowetting display devices and fabrication methods thereof

ABSTRACT

Electrowetting display devices and fabrication methods thereof are presented. The electrowetting display device includes a first substrate and a second substrate with a polar fluid layer and a non-polar fluid layer insolvable to each other and interposed between the first and second substrates. A first transparent electrode is disposed on the first substrate. A second electrode is disposed on the second substrate. A dielectric layer is disposed on the second electrode. A hydrophilic partition wall structure is directly disposed on the dielectric layer defining a plurality of pixel regions. A layer of low surface energy material is disposed on the dielectric layer within each of the pixel region.

This application is a divisional of U.S. application Ser. No.12/240,622, filed Sep. 29, 2008, (now U.S. Pat. No. 7,746,540), theentire disclosure of which is hereby incorporated by reference.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from aprior Taiwanese Patent Application No. 096150822, filed on Dec. 28,2007, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to display devices, and in particular toelectrowetting display devices and fabrication methods thereof.

2. Description of the Related Art

Electrowetting display devices are rendered images in accordance withelectrowetting or electrocapillary. Briefly, the free surface energy ofsome fluids is changed due to electric field effects such thatdistribution area of the fluids can thus change along with the electricfield effects.

PCT publication No. WO 2005/051091, the entirety of which is herebyincorporated by reference, discloses a structure of an electrowettingdisplay device. Referring to FIG. 1, a conventional electrowettingdisplay device comprises a second substrate 3 and a first substrate 4opposing to each other. A patterned pixel electrode 7 is disposed on thesecond substrate 3. A dielectric layer 8 such as a material layer with ahydrophobic surface is disposed on the second substrate 3 and the pixelelectrode 7. Patterned hydrophilic bank structures 13 (such as pixelwalls) are disposed on the dielectric layer 8 defining each pixelregion. An opaque non-polar fluid 5 including black dyes is disposed inthe pixel region between bank structures 13. A transparent polar fluid 6is disposed between the gap between the first substrate 4 and the secondsubstrate 3. When the operatic voltage is off, the opaque non-polarfluid 5 is uniformly distributed within each pixel region, thus thedisplay status is a dark state.

When the operatic voltage is on, i.e., an electric field is generatedbetween the first substrate 4 and the second substrate 3 by a voltagesource 9, the opaque non-polar fluid 5 is cohered due to anelectrowetting effect, thereby exposing most of the pixel region. Thus,the display status is a bright state.

The bank structure of the conventional electrowetting display device isa hydrophilic structure which is directly formed on the dielectric layer8 with hydrophobic low-surface-energy. It is beneficial that thelow-surface-energy material can be entirely applied to and formed on thesubstrate. However, it is difficult to fabricate the hydrophilic bankstructure 13 directly on the dielectric layer 8. The hydrophilic bankstructure 13, moreover, is prone to peel off from the low-surface-energydielectric layer 8 causing display failure.

U.S. Pub. No. 2007/0188676, the entirety of which is hereby incorporatedby reference, discloses an electrowetting display device. Coherence ofthe opaque non-polar fluid is controlled by an operatic electric fielddue to the electrowetting effect, thereby displaying a bright-state anda dark-state, respectively. FIG. 2 is a cross section of anotherconventional electrowetting display device. Referring to FIG. 2, theconventional electrowetting display device includes a back light unit 20and an electrowetting display 50 serving as a light switch. Theelectrowetting display 50 includes a second substrate 22, a pixelelectrode 24 disposed on the second substrate 22, and a dielectric layer(with a hydrophobic surface) 26 disposed on the pixel electrode 24.Patterned hydrophilic bank structures 28 are disposed on the dielectriclayer 26 defining each pixel region. An opaque non-polar fluid 45 withblack dyes and a transparent polar fluid 40 are disposed within eachpixel region. The first substrate 30 with patterned common electrodes 32disposed thereon is arranged on the bank structure 28 and transparentpolar fluid 40 opposing to the second substrate 22.

Furthermore, PCT publication No. WO 2006/017129, the entirety of whichis hereby incorporated by reference, discloses a transflectiveelectrowetting display structure in which a second substrate and a firstsubstrate attached with color filters are assembled. A polar fluid and ablack non-polar fluid are interposed between the second and firstsubstrate. The transflective color electrowetting display includes aplurality of pixels. Each pixel is divided into a transmission regionand a reflective region on the second substrate.

FIG. 3 is a cross section of a conventional transflective electrowettingdisplay. Referring to FIG. 3, the conventional transflective colorelectrowetting display includes a second substrate 112 disposed on aback light unit 111. A patterned reflector 113 corresponding to thereflective region of each pixel is disposed on the second substrate 112.A second transparent electrode 114 is disposed on the entire region ofthe second substrate 112 covering the reflector 113. A dielectric layer(with hydrophobic surface characteristics) 115 is disposed on the secondtransparent electrode 114. A patterned bank structure 116 is disposed onthe dielectric layer 115 defining a plurality of pixel regions.

A first substrate 118 is opposing to the second substrate 112. A firsttransparent electrode 117 is disposed on the first substrate 118. Atransparent polar fluid layer 121 and opaque non-polar fluid layers 120a-120 c are interposed in the gap between the second substrate 112 andthe first substrate 118. A power supply applies a bias between the firsttransparent electrode 114 and the second transparent electrode 117. Anelectrowetting force due to the bias causes convergence of the opaquenon-polar fluid layer, thereby controlling reflective and transmissiveregions of each pixel operation. When the applied voltage exceeds thethreshold voltage, the opaque non-polar fluid layer begins to convergegradually exposing both the reflective and transmissive regions.

The second substrate structure of the conventional electrowettingdisplay primarily includes a material layer with low-surface energy anda patterned hydrophilic structure fabricated on the low-surface energylayer. However, since the surface of the low-surface energy layer hasanti-adhesion properties, it is difficult to proceed with subsequentlarge area application and processes. Moreover, even if a hydrophilicstructure is firmly formed on the low-surface energy layer, thehydrophilic structure will easily be peeled off from the low-surfaceenergy layer due to its surface characteristics. Therefore, there is aneed for a fabricating method, which firmly fabricates a hydrophilicstructure on a low-surface energy dielectric layer, overcoming problemsassociated with the surface characteristics of the hydrophilicstructure, so as to improve structural stability of an electrowettingdisplay device.

BRIEF SUMMARY OF THE INVENTION

The main features and key aspects of the invention are related tostructures of electrowetting display devices and fabrication methodsthereof. A low-surface energy dielectric layer is patterned so that thehydrophilic bank structure is directly formed between the patternedlow-surface energy layer, and the bottom of the hydrophilic bankstructure directly contacts the underlying dielectric layer. Thefabrication process of the hydrophilic structure is simplified, andstructural reliability is improved.

Embodiments of the invention provide an electrowetting display device,comprising: a first substrate and a second substrate opposing to eachother with a polar solution layer and a non-polar solution layerinterposed therebetween; a first transparent electrode disposed on thefirst substrate; a second electrode disposed on the second substrate; adielectric layer disposed on the second electrode; a bank structure witha hydrophilic surface directly disposed on the dielectric layer, whereina plurality of regions are defined by the bank structure; and a layer oflow-surface-energy material disposed on the dielectric layer within theplurality of regions.

Embodiments of the invention also provide an electrowetting displaydevice, comprising: a first substrate and a second substrate opposing toeach other with a transparent polar solution layer and an opaquenon-polar solution layer interposed therebetween; a first transparentelectrode disposed on the first substrate; a second electrode disposedon the second substrate; a dielectric layer disposed on the secondelectrode; a bank structure with a hydrophilic surface directly disposedon the dielectric layer, wherein a plurality of pixel regions aredefined by the bank structure; and a layer of low-surface-energymaterial disposed on the dielectric layer within the plurality of pixelregions.

Embodiments of the invention further provide a fabrication method for anelectrowetting display device, comprising: providing a first substrateand forming a first transparent electrode layer thereon; providing asecond substrate and forming a second electrode layer thereon; forming adielectric layer on the second electrode; forming a patterned layer oflow-surface-energy material on the dielectric layer exposing a portionof the dielectric layer; forming a bank structure with a hydrophilicsurface on the dielectric layer, wherein a plurality of regions aredefined by the bank structure exposing the patterned layer oflow-surface-energy material; and oppositely assembling the firstsubstrate and the second substrate with a polar solution layer and anon-polar solution layer sandwiched therebetween.

Embodiments of the invention still further provide a fabrication methodfor an electrowetting display device, comprising: providing a firstsubstrate and forming a first transparent electrode layer thereon;providing a second substrate and forming a second electrode layerthereon; forming a dielectric layer on the second electrode; forming abank structure with a hydrophilic surface on the dielectric layer,wherein a plurality of pixel regions are defined by the bank structure;forming a patterned layer of low-surface-energy material on thedielectric layer within the plurality of pixel regions; and oppositelyassembling the first substrate and the second substrate with a polarsolution layer and a non-polar solution layer sandwiched therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by reading the subsequentdetailed description and examples with references made to theaccompanying drawings, wherein:

FIG. 1 is a cross section of a conventional electrowetting displaydevice;

FIG. 2 is a cross section of another conventional electrowetting displaydevice;

FIG. 3 is a cross section of a conventional transflective electrowettingdisplay;

FIG. 4A and FIG. 4B are comparisons respectively illustrating aconventional second substrate structure and an embodiment of the secondsubstrate structure of the electrowetting display device of theinvention;

FIG. 5 is a fabrication flowchart of an embodiment of the electrowettingdisplay device of the invention;

FIGS. 6A-6E are cross sections respectively illustrating eachfabrication step of an embodiment of the electrowetting display deviceof the invention;

FIGS. 7A-7D are schematic views illustrating various exemplary bankstructures with hydrophilic surfaces according to embodiments of theinvention;

FIG. 8 is a fabrication flowchart of another embodiment of theelectrowetting display device of the invention;

FIGS. 9A-9E are cross sections respectively illustrating eachfabrication step of another embodiment of the electrowetting displaydevice of the invention; and

FIG. 10 is a cross section of further another embodiment of theelectrowetting display device of the invention.

DETAILED DESCRIPTION OF THE INVENTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof various embodiments. Specific examples of components and arrangementsare described below to simplify the present disclosure. These are merelyexamples and are not intended to be limiting. In addition, the presentdisclosure may repeat reference numerals and/or letters in the variousexamples. This repetition is for the purpose of simplicity and clarityand does not in itself indicate a relationship between the variousembodiments and/or configurations discussed. Moreover, the formation ofa first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact or not in direct contact.

FIG. 4A and FIG. 4B are comparisons respectively illustrating aconventional second substrate structure and an embodiment of the secondsubstrate structure of the electrowetting display device of theinvention. Referring to FIG. 4A, the second substrate structure of theconventional electrowetting display includes a second substratestructure with a dielectric layer (not shown) thereon. A material layerof low-surface-energy (or with hydrophobic surface characteristics) 220Ais entirely and blanketly formed on the second substrate structure 210.Next, a bank structure 230A with a hydrophilic surface is formed on thelow-surface-energy material layer 220A. However, as mentionedpreviously, since the surface of the low-surface-energy material layer220A has anti-adhesion properties, the interface 225A between thelow-surface-energy material layer 220A and the bank structure 230A cannot create desirable adhesion due to polarity differences such that itis difficult to proceed with large area applications and subsequentprocesses. Moreover, even if a hydrophilic structure is firmly formed onthe low-surface energy layer 220A, the hydrophilic structure will easilybe peeled off from the dielectric layer.

Conversely, referring to FIG. 4B, an embodiment of the second substratestructure of the electrowetting display includes a second substratestructure 210 with other active devices, passive devices, pixelelectrodes and a dielectric layer (not shown) covering thereon. Apatterned low-surface-energy material layer (or a layer with hydrophobicsurface characteristics) 220B is formed on the second substratestructure 210 exposing predetermined intervals of surfaces of the secondsubstrate structure. Next, a bank structure 230B with a hydrophilicsurface is formed between the patterned low-surface-energy materiallayers 220B. Therefore, the bank structure 230B is prevented from notdirectly forming on the low-surface-energy material layer 220B. Theinterface between the second substrate structure 210 and the bankstructure 230B has excellent adhesion because of polar compatibility,thereby improving reliability of the electrowetting display device.

FIG. 5 is a fabrication flowchart of an embodiment of the electrowettingdisplay device of the invention. Referring to FIG. 5, a fabricationmethod for the electrowetting display device includes providing a firstsubstrate (step S310) and forming a first transparent electrode on thefirst substrate (step S312). Additionally, a second substrate isprovided (step S320). A second electrode is formed on the secondsubstrate (step S322). The second electrode can be a transparentelectrode or a reflective electrode. Next, a dielectric layer is formedon the second electrode (step S324), and a layer of patternedlow-surface-energy material is then formed on the dielectric layer (stepS326), corresponding to each pixel region with an interval portion ofthe surface of the dielectric layer exposed. Subsequently, hydrophilicbank structures are formed on the dielectric layer (step S328). The bankstructures are positioned on the interval portion of the exposed surfaceof the dielectric layer. The bank structures directly contact thedielectric layer, thereby defining a plurality of regions and exposingthe low-surface-energy material layer. Next, the first and secondsubstrates are oppositely assembled with a polar solution and anon-polar solution layer sandwiched therebetween (step S330).Subsequently, other essential fabrication steps such as assembling aback light unit and color filters are proceeded to complete theelectrowetting display device (step S340).

FIGS. 6A-6E are cross sections respectively illustrating eachfabrication step of an embodiment of the electrowetting display deviceof the invention. Referring to FIG. 6A, a second substrate 410 isprovided and is made of materials including glass, polymer or metal. Asecond electrode 420 is disposed on the second substrate 410. Next, adielectric layer 430 is formed on the second electrode 420.

The second electrode 420 is a patterned structure including arectangular, square, triangular, circular, trapezoid, or elliptic shape.The second electrode is made of metal or oxide with a thicknessapproximately in a range between 0.01 μm and 1 μm.

The dielectric layer 430 is made of silicon dioxide (SiO₂), siliconnitride (SiN_(x)), tantalum oxide (Ta₂O₃), lead zirconate titanate(PZT), barium strontium titanate (BST), barium titanate (BTO) orpolyvinylidene difluoride (PVDF).

Subsequently, a relief printing process is performed. The patternedlow-surface-energy material layer 445 is transferred on the dielectriclayer 430 by a roller 440 and a relief printing plate 442. The patternof the low-surface-energy material layer 445 corresponds to each pixelregion with an interval portion of the surface of the dielectric layer430 exposed, as shown in FIG. 6B. The low-surface-energy material layer445 comprises fluoride-containing or chloride-containing hydrophobicpolymer materials, or fluoride-containing or chloride-containingself-assembly monolayer films. The thickness of the low-surface-energymaterial layer is approximately in a range between 0.1 nm and 1 μm andthe surface energy of the low-surface-energy material layer isapproximately 25 dyne/cm, preferably less than 20 dyne/cm.

Referring FIG. 6C, a thick-film material layer 450 such as a positivephotoresist, a negative photoresist, a photoset resin or a thermosetresin is entirely and blanketly formed on the dielectric layer 430covering the low-surface-energy material layer 445. Subsequently, alithography process is performed to pattern the thick-film materiallayer 450, thereby creating a bank structure 450 with a hydrophilicsurface. The bank structure 450 is positioned on the interval portion ofthe exposed surface of the dielectric layer 430. The bank structure 450directly contacts the dielectric layer 430, thereby defining a pluralityof regions and exposing the low-surface-energy material layer 445, asshown in FIG. 6D.

Note that the bank structure 450 can be a black matrix bank structurecapable of absorbing visible lights. The thickness of the bank structureis approximately in a range between 5 μm and 50 μm.

Referring to FIG. 6D, on the other hand, a first substrate 460 isprovided. A transparent electrode 470 is formed on the first substrate460. The first substrate 460 is made of glass, polymer or metal. Thefirst transparent electrode 470 is a patterned structure comprising arectangular, square, triangular, circular, trapezoid, or elliptic shape.Alternatively, the transparent electrode 470 can be an entirely blanketelectrode structure. Moreover, the first transparent electrode 470 ismade of metal or oxide with a thickness approximately in a range between0.01 μm and 1 μm.

Subsequently, the second substrate 410 and the first substrate 460 areoppositely assembled with a non-polar solution layer 480 and a polarsolution layer 485 interposed therebetween, as shown in FIG. 6E. Thenon-polar solution layer 480 can be made of silicon oil, decane,dodecane, tetradecane, or any combinations thereof. The thickness of thenon-polar solution layer is approximately in a range between 1 μm and 10μm. The non-polar solution layer 480 can further comprise a color dye ora color pigment. On the other hand, the polar solution layer 485 can bemade of water, sodium chloride solution, or potassium chloride solutionwith a thickness approximately in a range between 10 μm and 100 μm.

FIGS. 7A-7D are schematic views illustrating various exemplary bankstructures with hydrophilic surfaces according to embodiments of theinvention. These bank structures are exemplified merely forillustration, and are not limited thereto. Referring to FIG. 7A, thebank structure 450A with a hydrophilic surface can be a striped ridgeshape with a uniform width which is equal to that of intervals of thepatterned low-surface-energy material layer 445. Referring to FIG. 7B,according to another embodiment, the bank structure 450B with ahydrophilic surface can directly contact the dielectric layer 430 andhave two flank extension portions contacting the low-surface-energymaterial layer 445. The top of the bank structure 450B is wider than orhas the same width as the bottom of the bank structure 450B.Alternatively, referring to FIG. 7C, the bank structure 450C with ahydrophilic surface can directly contact the dielectric layer 430 andhave a cross section of a trapezoid, i.e., the top of the bank structure450C is narrower than the bottom of the bank structure 450C. Referringto FIG. 7D, the bank structure 450D with a hydrophilic surface can be acheckered pattern. Alternatively, the shape of the low-surface-energymaterial layer 445 is striped, and the shape of the bank structure 450Dis a checkered pattern. The bank structure 450D with a hydrophilicsurface directly contacts the dielectric layer 430 along the directionof the stripes, and the bank structure 450D directly contacts thelow-surface-energy material layer 445 crossing the direction of thestripes.

FIG. 8 is a fabrication flowchart of another embodiment of theelectrowetting display device of the invention. The fabrication steps ofthis embodiment of the electrowetting display device are nearlyidentical to the abovementioned fabrication steps of the electrowettingdisplay in FIG. 5, and for simplicity their detailed description isomitted. The embodiment of FIG. 8 is different from the embodiment ofFIG. 5 in that the bank structure with a hydrophilic surface is formedin advance on the dielectric layer (step S328) defining a plurality ofpixel regions and exposing a portion of the dielectric layer.Subsequently, a layer of patterned low-surface-energy material is thenformed on the dielectric layer (step S326). For example,fluoride-containing hydrophobic polymer materials are sprayed by inkjetprinting on the dielectric layer within each pixel region.

FIGS. 9A-9E are cross sections respectively illustrating eachfabrication step of another embodiment of the electrowetting displaydevice of the invention. Referring to FIG. 9A, a second substrate 410 isprovided and is made of including glass, polymer or metal. A secondelectrode 420 is disposed on the second substrate 410. Next, adielectric layer 430 is formed on the second electrode 420.

Referring to FIG. 9B, a thick-film material layer 450 such as a positivephotoresist, a negative photoresist, a photoset resin or a thermosetresin is entirely and blanketly formed on the dielectric layer 430.Subsequently, lithography and etching processes are performed to patternthe thick-film material layer into a bank structure 450 with ahydrophilic surface.

Next, referring to FIG. 9C, an inkjet process is performed by an inkjetprinting apparatus 500 to inject fluoride-containing hydrophilic polymermaterials 445′ on the dielectric layer 430 within each pixel region tocreate patterned low-surface-energy material layer 445.

Referring to FIG. 9D, on the other hand, a first substrate 460 isprovided. A first transparent electrode 470 is formed on the firstsubstrate 460. Subsequently, the second substrate 410 and the firstsubstrate 460 are oppositely assembled with a non-polar solution layer480 and a polar solution layer 485 interposed therebetween, as shown inFIG. 9E. Subsequently, other essential fabrication steps such asassembling a back light unit and color filters are proceeded to completethe electrowetting display device.

Note that although in the abovementioned embodiments, the bank structure450 with a hydrophilic surface is exemplified to directly contact thefirst substrate 460, it is not limited to the electrowetting displaydevices of the invention. In other embodiments, the height of the bankstructure 450 a with a hydrophilic surface is less than the cell gapbetween the first substrate and the second substrate so that the bankstructure 450 a is not in contact with the first substrate 460, as shownin FIG. 10. For example, the height of the bank structure 450 a is about10 μm, and the cell gap between the first substrate and the secondsubstrate is about 50 μm.

While the invention has been described by way of example and in terms ofthe several embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangements (aswould be apparent to those skilled in the art). Therefore, the scope ofthe appended claims should be accorded the broadest interpretation so asto encompass all such modifications and similar arrangements.

1. A fabrication method for an electrowetting display device,comprising: providing a first substrate and forming a first transparentelectrode layer thereon; providing a second substrate and forming asecond electrode layer thereon; forming a dielectric layer on the secondelectrode; forming a patterned layer of low-surface-energy material onthe dielectric layer exposing a portion of the dielectric layer; forminga bank structure with a hydrophilic surface on the dielectric layer,wherein a plurality of regions are defined by the bank structureexposing the patterned layer of low-surface-energy material; andoppositely assembling the first substrate and the second substrate witha polar solution layer and a non-polar solution layer sandwichedtherebetween.
 2. The fabrication method as claimed in claim 1, whereinthe bank structure comprises two flank extension portions contacting thelayer of low-surface-energy material.
 3. The fabrication method asclaimed in claim 1, wherein the bank structure is an array of continuousbank structures with at least one portion directly disposed on thedielectric layer.
 4. The fabrication method as claimed in claim 1,wherein the bank structure is made of a positive photoresist, a negativephotoresist, a photoset resin or a thermoset resin.
 5. The fabricationmethod as claimed in claim 1, wherein the bank structure is a blackmatrix bank structure capable of absorbing visible lights.
 6. Thefabrication method as claimed in claim 1, wherein formation of the bankstructure comprises a lithography, molding, screen printing, dry filmtransferring, or laser heating transferring process.
 7. The fabricationmethod as claimed in claim 1, wherein the first substrate or the secondsubstrate is made of glass, polymer or metal.
 8. The fabrication methodas claimed in claim 1, wherein the first transparent electrode and thesecond transparent electrode are patterned structures comprising arectangular, square, triangular, circular, trapezoid, or elliptic shape.9. The fabrication method as claimed in claim 1, wherein the firsttransparent electrode or the second electrode is made of metal or oxide.10. The fabrication method as claimed in claim 1, wherein formation ofthe layer of low-surface-energy material comprises a relief (anastatic)printing, lithography, screen printing, inkjet printing, dry filmtransferring, micro contact printing, or laser heating transferringprocess.
 11. The fabrication method as claimed in claim 1, wherein thelayer of low-surface-energy material comprises fluoride-containing orchloride-containing hydrophobic polymer materials, orfluoride-containing or chloride-containing self-assembly monolayerfilms.
 12. The fabrication method as claimed in claim 1, wherein thethickness of the layer of low-surface-energy material is approximatelyin a range between 0.1 nm and 1 μm, and wherein the surface energy ofthe layer of low-surface-energy material is approximately 25 dyne/cm.13. The fabrication method as claimed in claim 1, wherein the polarsolution layer is made of water, sodium chloride solution, or potassiumchloride solution.
 14. The fabrication method as claimed in claim 1,wherein the non-polar solution layer is made of silicon oil, decane,dodecane, tetradecane, or any combinations thereof.
 15. The fabricationmethod as claimed in claim 1, wherein the non-polar solution layercomprises a color dye or a color pigment.
 16. The fabrication method asclaimed in claim 1, wherein the dielectric layer is made of silicondioxide (SiO2), silicon nitride (SiNx), tantalum oxide (Ta2O3), leadzirconate titanate (PZT), barium strontium titanate (BST), bariumtitanate (BTO) or polyvinylidene difluoride (PVDF).